1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens. The zoom lens is suitable to, for example, digital cameras, video cameras, silver-halide film cameras, monitoring cameras, television cameras, and the like.
2. Description of the Related Art
As image pickup apparatuses, digital single-lens reflex cameras (D-SLRs) are widely used. D-SLRs include camera bodies that include image pickup sensors such as a charge-coupled device (CCD) sensor or complementary metal-oxide semiconductor (CMOS) sensor, and image pickup lenses that form optical images on surfaces of image pickup sensors. Today, the D-SLRs have a function of taking movies in addition to a function of taking still images. For an automatic focusing operation in taking movies with a D-SLR, a high-frequency detecting method, which is called a contrast detection auto-focus method (contrast detection AF method), is widely used. The contrast detection AF method evaluates an in-focus state of an image taking lens by detecting a high-frequency component in an imaging signal.
In a D-SLR camera using the contrast detection AF method, the focus lens unit vibrates in an optical axis direction at a high speed (hereinafter referred to as “wobbling”) in order to detect a direction of deviation from an in-focus state. A signal component in a particular frequency band of an image area is detected from an output signal from an image pickup sensor, and an optimal position of the focus lens unit where the image taking lens is in-focus is calculated. After that, the focus lens unit is moved to the optimal position and focusing is complete.
When a movie is taken, in order to prevent an uncomfortable sensation such as flickering from occurring, the focus lens unit needs to be driven at a high speed during wobbling. Thus, when the focus lens unit is large in size, a motor and an actuator that drive the focus lens unit at a high speed become larger, thereby increasing the size of an image pickup apparatus.
As the image taking lens used for the D-SLR, there is a demand for a zoom lens having a wide field angle and a long back focus distance. To meet this demand, a negative-lead zoom lens is known. In a negative-lead type zoom lens, a lens unit having a negative refractive power is disposed on the side closest to the object to be imaged. As a negative-lead zoom lens, a wide field angle zoom lens is known, which facilitates high-speed focusing by moving part of a first lens unit having a negative refractive power in the optical axis direction.
Japanese Patent Laid-Open No. 2002-311330 discloses a zoom lens that includes, sequentially from the object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power, in which a cemented lens element, which is part of the first lens unit, is used as a focus lens unit.
U.S. Pat. No. 5,831,773 discloses a zoom lens that includes, sequentially from the object side to the image side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, in which a negative lens element on the side closest to the object in the first lens unit is used as the focus lens unit.
The negative-lead zoom lens, which has an asymmetrical lens structure relative to an aperture stop, tends to exhibit large variations in aberrations in focusing. In particular, when focusing is performed by moving a small and right lens unit that is part of the first lens unit having a negative refractive power in order to make automatic focusing speed be faster, variations in chromatic aberration increases. This makes it difficult to achieve high optical performance in a whole range of the object distance.
Thus, in order to ensure a long back focus distance, enlarge a field angle, and achieve high-speed automatic focus detection, it is important to suitably set a lens structure of the first lens unit, which has a negative refractive power and includes a focus lens unit.